The present invention relates to an apparatus and method for optically processing optical signals for the purposes of detecting the presence of a digital bit pattern therein.
In a wide range of applications in optical data transmission it is required to detect a particular digital bit pattern within a high bit-rate optical signal. An example of this is the identification of a packet header in an optically transmitted data packet (e.g. an IP datagram), for subsequent switching or routing of that data packet. Ideally, this identification should be performed optically, that is to say, without recourse to opto-electrical conversion of the packet header.
It is well known that the act of converting an optical packet header into the electrical domain, prior to processing (e.g. routing or switching) of that optical packet, reduces the overall speed of processing. In particular the routing performance of IP routers is degraded when opto-electrical packet header identification is required and results in a transmission bottleneck at the router.
The bottleneck results at least in part from the time required to perform opto-electrical conversion of a given packet header and, as a consequence of this, the header of an optical data packet typically has to be transmitted at a lower bit rate than the payload with which it is associated. The lower bit rate of the header allows more time to be spent in reading the header data opto-electrically, but occupies a relatively large percentage of the optical packet slot thereby lowering bandwidth efficiency.
Clearly, the all-optical identification of optical packet headers would be advantageous in overcoming the aforementioned deficiencies in existing techniques.
One such all-optical technique that has been proposed employs the phase-modulation (binary shift-keying) of the optical carrier wave of a data packet to map digital routing information onto the header of that packet. In particular, the optical carrier wave associated with the header part of an optical data packet is phase-shifted by either 0 degrees or 180 degrees over a number of successive intervals (i.e. a string) of the same fixed duration (e.g. 5 ps). This encodes routing information into the optical header in terms of the pattern of the sequence of carrier phases in the aforementioned string of intervals.
A proposed optical processor is then able to recognize such header information by splitting a received phase-modulated header signal into as many copies as there are intervals in the string it seeks to identify. Each copy is then phase shifted in a predetermined manner and then correlated with every other phase-shifted copy. A correlation peak emerges when the received string (sequence of carrier phases) matches the one that the processor seeks to identify. Thus, by performing optical correlation of the phase-modulated radiation associated with a packet header, the processor may identify a predetermined header and control optical routing of the data packet accordingly. However, implementation of such a technique requires the use of optical phase-modulation in the encoding of header data into a data packet. Optical amplitude-modulation of a packet carrier wave is more typically used to encode not only the header data but also the payload data of most optical data packets in the art. Clearly, the aforementioned optical processor would therefore be unable to process such amplitude-modulated headers. Furthermore, optical phase-modulation tends to be difficult to control and the phase-shifters employed in the proposed optical processor are typically highly sensitive to bit-rate changes in the digital data that phase-modulation is being used to convey. Amplitude modulators tend to be more robust to bit-rate changes, are simple to control and are often cheaper to implement.
Consequently, a need has been identified for an optical signal processor able to recognize the presence of a digitally amplitude-modulated optical signal, in particular an amplitude-modulated optical packet header.
The present invention aims to overcome at least some of the aforementioned deficiencies in the prior art.
According to a first aspect of the present invention there is provided an optical signal processor for the optical processing of a digitally amplitude-modulated optical signal input thereto, to determine the presence or absence of a predetermined digital bit-pattern within said optical signal, the optical signal processor possessing optical processing means including;
signal duplicating means operable to derive from said input digital optical signal one or more duplicate digital optical signals which are each a duplicate of the digital bit-patterns present within the input optical signal;
signal duplicating and inverting means operable to derive from said input digital optical signal one or more polarity-inverted duplicate digital optical signals, the polarity of each bit of which is the inverse of that of the signal bit from which it is derived, wherein;
the optical processing means is operable to derive from said duplicate signals and said polarity-inverted duplicate signals an optical detection signal which indicates the presence or absence of said predetermined amplitude-modulated digital bit-pattern within the input digital signal processed thereby.
Thus, the present invention according to its first aspect provides a means via which digitally amplitude-modulated optical bit-patterns, such as the header of an optical data packet, may be detected within an optical signal without recourse to prior opto-electrical conversion thereof. Clearly, by obviating the need to convert optical signals into equivalent electrical signals prior to signal processing/detection, the present invention enables a substantial reduction in the delay associated with detection of amplitude-modulated optical bit-patterns.
Furthermore, the present invention obviates the need to employ the techniques of optical phase-modulation to optically encode digital data in an attempt to permit detection thereof without opto-electrical conversion. Consequently, the present invention provides an optical processor which aims to overcome at least some of the drawbacks and limitations inherent in employing such phase-modulation.
Preferably, the optical processing means of the present invention according to its first aspect, further includes optical transmission delay means and is operable to transmit through said delay means said duplicate signals and said polarity-inverted duplicate signals derived from said input digital optical signal, wherein;
the optical processing means is operable to derive said optical detection signal from a combination of said duplicate signals and said polarity-inverted duplicate signals output of said optical delay means.
These optical delay means may preferably comprise optical fibre delay lines through which optical signal radiation, derived from the input digital signal, may be transmitted. The delay means may alternatively or additionally include one or more fibre recirculating loops whereby signal radiation is circulated within a fibre loop until sufficient delay has been accumulated whereupon the signal is output of the loop (gated), as is known in the art. Furthermore, the delay means may employ optical memory means (write/read) such as regenerative loop memories.
Furthermore, it is preferable that the signal duplicating means and the signal duplicating and inverting means are operable to produce one or more duplicate signals (polarity-inverted or otherwise) each one of which is conveyed via optical radiation of a wavelength different from that of the radiation conveying any of the other signal duplicates produced thereby. Alternatively, two or more (e.g. all) of the duplicate signals produced by any one (or both) of the signal duplicating means and the signal duplicating and inverting means, may be of the same wavelength.
The optical processing means preferably includes a plurality of separate optical transmission delay means and is operable to transmit respective ones of said duplicate signals and said polarity-inverted duplicate signals through respective individual ones of the separate optical delay means, wherein;
the optical processing means is operable to derive said optical detection signal from a combination of the delayed duplicate signals and delayed polarity-inverted duplicate signals output of said optical delay means.
More preferably, the optical transmission delay provided by each one of said plurality of transmission delay means differs from that provided by any of the other transmission delay means within that plurality by an amount equal to an integer multiple of the optical bit period associated with said predetermined amplitude-modulated digital bit-pattern. More preferably, the delay associated with each individual delay means is unique to that delay means, and preferably the unique delays of successive ones of the delay means successively increase by one said bit period.
Thus, for example, a plurality of respective duplicate optical signals simultaneously input to respective individual ones of the transmission delay means will be subsequently output thereof successively bit-shifted by one bit period.
The optical processing means preferably includes signal routing means operable to direct respective ones of said duplicate signals and said polarity-inverted duplicate signals input thereto to respective individual ones of said separate optical delay means in dependance upon the wavelength of the radiation conveying each respective duplicate signal.
Thus, it will be understood that by using wavelength-dependant optical routing means to uniquely route the one or more duplicate signals (of any polarity) to predefined optical transmission delay means, the present invention permits the transmission delay imposed on each one duplicate signal to be predetermined by initially producing duplicates at the appropriate wavelength. Preferably the transmission delay means imposing this delay includes a plurality of separate optical transmission delay lines the optical transmission delay provided by each one of which is determined by the length thereof.
Optical routing such as this may be achieved using an optical de-multiplexer, and preferably an arrayed waveguide grating de-multiplexer (AWG). Alternatively, the optical signal routing means may comprise one or more wavelength-selective optical splitters/couplers via which said duplicate signals (polarity-inverted or otherwise) may be routed to selected ones of the plurality of transmission delay means, as will be readily apparent to the skilled person.
Alternatively, in embodiments of the present invention where some or all of said duplicate signals (of any polarity) are conveyed via the same wavelength, the optical signal routing means may alternatively or additionally include non-wavelength-selective optical signal routing elements. Such elements may consist of one or more 1-to-N (e.g. xe2x80x9cTreexe2x80x9d) couplers and/or optical splitters and the like via which some or all of said duplicate signals may be routed to selected ones of the plurality of transmission delay means independently of the wavelength of those signals.
Preferably, the optical processing means includes optical combining means for combining in equal proportion the delayed duplicate and polarity-inverted duplicate signals output from each one of said transmission delay means so as to thereby derive said optical detection signal. Preferably, the delayed signals are simultaneously combined by an optical combiner onto one optical transmission line.
The signal duplicating and inverting means preferably includes a semiconductor optical amplifier (SOA) operable, via the process of Cross-Gain Modulation, to output an optical signal which is the polarity-inverted duplicate of optical signal radiation input thereto. Additionally, the signal duplicating means preferably includes two concatenated semiconductor optical amplifiers (SOA) each operable, via the process of Cross-Gain Modulation, to output an optical signal which is the polarity-inverted duplicate of optical signal radiation input thereto.
The signal processor according to the first aspect of the present invention is preferably operable to partition optical signal radiation input thereto into two portions and to transmit a first such portion via said optical processing means, to a first optical output port thereof and to transmit the second such portion via an optical transmission delay line, to a second optical output of the optical signal processor wherein;
the transmission delay provided by said delay line is such as to permit substantially simultaneous arrival of the optically processed first portion and the delayed second portion at said first and second optical output ports.
Thus, the transmission delay line in question enables the delay associated with optically processing the first portion to be accounted for in the delay imposed upon the second portion.
According to a second aspect of the present invention, there is provided a method for the optical processing of a digitally amplitude-modulated optical signal so as to determine the presence or absence of a predetermined digital bit-pattern therein, the optical signal processing method including the steps of;
deriving from said digital optical signal one or more duplicate digital optical signals which are each a duplicate of the digital bit-patterns present within the optical signal;
deriving from said digital optical signal one or more polarity-inverted duplicate digital optical signals, the polarity of each bit of which is the inverse of that of the signal bit from which it is derived;
deriving from said duplicate signals and said polarity-inverted duplicate signals an optical detection signal which indicates the presence or absence of said predetermined amplitude-modulated digital bit-pattern within the digital optical signal.
Preferably, the method according to the second aspect of the invention includes the step of transmitting through an optical transmission delay means said duplicate signals and said polarity-inverted duplicate signals derived from said digital optical signal, and;
deriving said optical detection signal from a combination of said duplicate signals and said polarity-inverted duplicate signals output of said optical delay means.
Preferably, said duplicate signals and said polarity-inverted duplicate signals are each conveyed via optical radiation of a wavelength different from that of the radiation conveying any of the other signal duplicates.
More preferably, respective ones of said duplicate signals and said polarity-inverted duplicate signals are transmitted through respective individual ones of a plurality of separate optical transmission delay means, wherein;
said optical detection signal is derived from a combination of the delayed duplicate signals and the delayed polarity-inverted duplicate signals output of said optical delay means.
It is further preferable that each one of said separate transmission delay means provides a transmission delay which differs from that provided by any of the other transmission delay means by an amount equal to an integer multiple of the optical bit period associated with said predetermined amplitude-modulated digital bit-pattern.
In preferred embodiments of the invention respective ones of said duplicate signals and said polarity-inverted duplicate signals are directed to respective individual ones of said separate optical delay means in dependance upon the wavelength of the radiation conveying each respective duplicate signal.
The method may preferably further include the step of combining in equal proportion the delayed duplicate and polarity-inverted duplicate signals output from each one of said transmission delay means so as to thereby derive said optical detection signal.
In preferred embodiments the signal duplicating and inverting is provided via the process of Cross-Gain Modulation of a semiconductor optical amplifier (SOA) operable to output an optical signal which is the polarity-inverted duplicate of optical signal radiation input thereto. Furthermore, signal duplicating is preferably provided via the process of Cross-Gain Modulation two concatenated semiconductor optical amplifiers (SOA) each operable to output an optical signal which is the polarity-inverted duplicate of optical signal radiation input thereto.
Suitably, the method according to the second aspect of the present invention additionally includes the steps of;
partitioning optical signal radiation input thereto into two portions and optically processing a first of said two portions and transmitting said second such portion via an optical transmission delay line, wherein;
the transmission delay of said second portion is substantially equal to the delay associated with the optical processing of said first portion.